In general terms, medical valving devices often act as a sealed port that may be repeatedly accessed to non-invasively inject fluid into (or withdraw fluid from) a patient's vasculature. Consequently, a medical valve acting as a fluid port permits the patient's vasculature to be freely accessed without requiring such patient's skin be repeatedly pierced by a needle.
More specifically, after it is inserted into a medical valve, a syringe may freely inject or withdraw fluid from the patient. Problems arise, however, when the syringe is withdrawn from the valve. Specifically, a back pressure (i.e., a proximally directed pressure) produced by the withdrawing syringe undesirably can draw blood proximally into the valve or a catheter connected to the valve. In addition to coagulating and impeding the mechanical operation of the valve, blood in the valve also compromises the sterility of the valve and/or catheter.